fabric flow 1-8-08 - Flow Through Fabrics Douglas J....

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1 Flow Through Fabrics Douglas J. Gardner Professor Wood-Polymer Hybrid Composites Fabric Preform Issues • Permeability • Compressibility • Ease of handling • Drapeability • Strength, stiffness and fiber volume fraction
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2 Darcy’s Law L p k A q v Δ Δ = = η ' ' v’ = velocity cm/s q’ = flow rate cm 3 /sec A = cross section cm 2 η = viscosity cps k = permeability cm 3 flow/sec(cps)(cm length)/(cm 2 (area))(atm pressure drop) •p = p r e s s u r e a t m L = length cm Darcy Units •K
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3 Permeability as a function of weave type Open weave results in higher porosity and higher permeability. Closed weave mats are difficult to penetrate, but offer higher strength and moduli. Effect of preform structure on flow is represented by permeability, K. The more porous or open preform structures have higher values of K and are easier to fill. Permeability may have a directional character • For anisotropic preforms, three values for the permeability may be required –K xx ,K yy , K zz • For isotropic preforms, one value is sufficient. –K = K xx =K yy
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4 Permeability Issues • Permeability, K, is dependent upon – Preform architecture – Resin viscosity • Permeability determines key processing parameters – Fill time – Injection pressure Compressibility
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5 Compressibility • The closing of the mold compresses the fabric and changes its architecture. A number of changes may occur: – Changes in the amount of nesting between layers – Changes in fiber packing within individual bundles--the aspect ratio of the fiber bundles can change – Fiber re-orientation Relationship between compaction pressure and the resulting fiber volume fraction in a preform
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6
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7 Anisotropic Flow • The ratio of in plane permeability of a fiber mat to its transverse permeability is defined as the anisotropy ratio. – Kxx and Kyy denote the in-
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fabric flow 1-8-08 - Flow Through Fabrics Douglas J....

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